Increased bone resorption in malignancy is often associated with hypercalcemia and metastatic disease, major health care problems. Tamoxifen, a triphenylethylene antiestrogen frequently used to treat breast cancer patients, paradoxically has both bone-preserving and hypocalcemic properties. Our studies indicate that tamoxifen inhibits bone resorption by osteoclasts by a mechanism distinct from the estrogen receptor, involving inhibition of the H+-ATPase proton pump at the specialized ruffled membrane, the site bone resorption. The inhibition of bone resorption by osteoclasts and of acid secretion by isolated osteoclast membranes by tamoxifen parallel in all respects inhibition by calmodulin antagonists, further suggesting that a calmodulin-dependent step is at least partially responsible. The goal of these investigations is to identify the molecular site or sites of action of tamoxifen in osteoclasts. The specific aims are; SPECIFIC AIM I: IDENTIFY THE SITES OF ACTION OF TAMOXIFEN IN INTACT OSTEOCLASTS. Part A. Characterize the effects of tamoxifen treatment of osteoclasts on acid transport in subsequently isolated membrane vesicles. Part B. Characterize the binding of tamoxifen to osteoclast proteins. Part C. Characterize the effects of tamoxifen on the subcellular distribution and concentration of proteins that mediate acid secretion. Part D. Characterize the effects of tamoxifen on the phosphorylation of proteins in intact osteoclasts. SPECIFIC AIM II; IDENTIFY THE MECHANISM BY WHICH TAMOXIFEN INHIBITS ACIDIFICATION IN OSTEOCLAST MEMBRANE PREPARATIONS. Part A. Characterize the effects of taxoxifen and calmodulin antagonists on vesicle acidification. Part B. Characterize the effects of tamoxifen on the components of the vesicle acidification mechanism. Part C. Characterize the effect of tamoxifen on phosphorylation of proteins in isolated membranes. Part D. Identify and characterize tamoxifen binding proteins(s) in osteoclast membranes. Upon identifying the molecular mechanism of tamoxifen inhibition of bone resorption, it is anticipated that more focused pharmacotherapeutics will be developed that will be of benefit to a broad population of patients. This includes diseases which are prevalent in minorities and women. According to Healthy People 2000, 28% of all cancers in women are of breast origin which are of breast origin which are commonly associated with metastasis, increased bone resorption and hypercalcemia. Furthermore cancer deaths, in general, are more prevalent in blacks (male=288/100,000; female=132/100,000) than whites (male=158/100,000; female=110/100,000) and postmenopausal osteoporosis in women is the major cause of approximately 1.3 million bone fractures each year.